(1) Field of the Invention
The present invention relates to a method of manufacturing a semiconductor memory in which information is stored by applying voltage and causing current to flow between electrodes to induce a change of state.
(2) Description of the Related Art
Currently, two types of memories, namely, Dynamic Random Access Memories (DRAMs) and flash memories, are widely used as large-scale integrated storage elements.
DRAMs are what are called volatile memories which allow fast writing and reading but require power consumption for memory retention. Therefore, DRAMs are mainly used in short-term memories such as the main memory, and the like, of a computer. Flash memories are what are called nonvolatile memories which do not require power consumption for memory retention. These storage elements have the drawback of low-speed information writing and rewriting, and are thus mainly used in long-term memories for digital cameras and music players.
If a storage element which allows large-scale integration and has both high-speed and non-volatility can be put into practical use, the need to use separate elements for short-term memories and long-term memories would be eliminated. Using such an element would enable the realization of computers, and the like, that can be used immediately after power is switched ON. In view of this, significant research geared towards the realization of storage elements which allow large-scale integration and have both high-speed and non-volatility is currently being carried out.
It is considered that overcoming the drawbacks of DRAMS and flash memories to realize a large-scale integrated nonvolatile high-speed memory would require a memory having a structure and operating principle that are different from DRAMS and flash memories. At present, research on memories of various structures and operating principles is widely being carried out. These include the Resistance Random Access Memory (ReRAM) and spin-injection Magnetic Resistance Random Access Memory (MRAM).
Both ReRAMs and spin-injection MRAMs are devices having two electrodes. A common point between these devices is that information is stored as a difference in the electrical resistance between electrodes. Furthermore, these devices also share in common the point that writing, rewriting, and reading of information is performed by applying voltage between the electrodes.
The characteristic features of each of these memories shall by described below.
(ReRAM)
FIG. 21 is an outline structure diagram showing the structure of a ReRAM. As shown in FIG. 21, the ReRAM is an element having a structure in which a variable resistance film 2102 is disposed between two electrodes 2101 and 2103. Information is held as a difference of electrical resistance between the electrodes. In the case of a 1-element, 1-bit memory which is the most basic configuration, the two electrical resistance states of low-resistance state and high-resistance state are assigned to correspond to the information 0 and 1 (or 1 and 0) respectively. Writing/rewriting of information is performed by causing the electrical resistance between the electrodes to change to the electrical resistance value corresponding to the information. In other words, this is the act of lowering the electrical resistance or increasing the electrical resistance.
There are two types of ReRAMs, namely, the non-polar and the bipolar, depending on how electrical resistance changing is induced.
FIG. 22 is a graph showing operational characteristics of a non-polar ReRAM. As shown in FIG. 22, in the case of the non-polar type, transition from the high-resistance state to the low-resistance state is induced by applying voltage that is greater than a threshold voltage. This is one type of what is called a dielectric breakdown phenomenon. Transition from the low-resistance state to the high-resistance state is also induced by applying voltage that is greater than a certain voltage. However, a voltage 2201 with which the transition from the low-resistance state to the high-resistance state occurs, is a voltage 2202 which is lower than the transition from the high-resistance state to the low-resistance state. In the case of m the non-polar type, because of the difference in the resistance state of the element prior to transition, merely having the two types of transition threshold voltages makes it possible to induce either of the transitions, from high resistance to low resistance or from low resistance to high resistance, by voltage application in the same direction.
FIG. 23 is a graph showing operational characteristics of a bipolar ReRAM. As shown in FIG. 23, in the case of the bi-polar type, the inducing of transition to the low-resistance state by applying a voltage that is greater than the threshold voltage is the same as with the non-polar type. What is different is that the application direction of the voltage 2301 which induces the transition from high resistance to low resistance, and the application direction of the voltage 2302 which induces the transition from low resistance to high resistance are reversed. In other words, transition between states is controlled by separately applying voltages of two types, namely positive and negative, between the two electrodes.
In both cases of the non-polar type and the bipolar type, the electrical resistance of the element does not change even when a voltage that is lower than the threshold for effecting a transition in resistance state is applied to between the electrodes. Therefore, the electrical resistance value, that is, the stored information can be read without being destroyed, by applying a voltage that satisfies this condition and causing current to flow between the electrodes.
(Spin-Injection MRAM)
FIG. 24 is a diagram showing the structure of an MRAM. As shown in FIG. 24, the MRAM has a structure in which a thin-tunnel insulating film 2403 made of MgO and so on is disposed between two electrodes 2401 and 2402 which are made from a ferromagnet. In the same manner as the ReRAM, information is held as a difference of electrical resistance between the electrodes. Specifically, the two electrical resistance states of the low-resistance state and the high-resistance state are assigned to correspond to the information 0 and 1 (or 1 and 0) respectively. Writing/rewriting of information is performed by causing the electrical resistance between the electrodes 2401 and 2402 to change to the electrical resistance value corresponding to the information. In other words, this is the act of lowering the electrical resistance or increasing the electrical resistance.
In the MRAM, the electrical resistance is determined by the magnetic orientations of both electrodes 2401 and 2402. Resistance decreases when the magnetic orientations of the two electrodes 2401 and 2402 are parallel, and increases when the orientations are antiparallel. Normally, rewriting of information is performed by keeping the magnetic orientation of one of the electrodes fixed, and reversing the magnetic orientation of the other.
The spin-injection MRAM, which among MRAMs has a particularly suitable scheme for miniaturization, is characterized by performing this magnetic reversal by current injection. When the magnetic orientation is to be made parallel, a current that is greater than the threshold current is caused to flow from the side of the fixed-magnetization electrode towards the variable-magnetization electrode. Furthermore, when magnetic orientation is to be made anti-parallel, a current that is greater than the threshold current is caused to flow, conversely, from the side of the variable-magnetization electrode towards the fixed-magnetization electrode.
Since magnetic reversal does not occur at the threshold current or below, adjusting voltage so as to limit current to be equal to or less than the threshold current allows the electrical resistance, that is, information to be read, without destroying the stored information.
(Lowering Power Consumption)
Like the ReRAM and the spin-injection MRAM, in the case of a device in which writing/rewriting of information is performed by inducing resistance change through application of voltage and current injection between electrodes, direct current is generated with each writing or rewriting, and thus power is consumed. Therefore, there is a demand for the reduction of the power consumption of the device by lowering the voltage and amount of current needed for inducing resistance change.
(Variation)
Furthermore, with these devices, suppression of characteristics-variation is important. In the case of a storage element in which the writing and rewriting of information is performed by inducing resistance change between electrodes by applying voltage and causing current to flow between the electrodes, the electrical resistance needs to be detected for the reading of information. For this reason, it is necessary to apply voltage and cause current to flow between the electrodes.
However, when the applied voltage exceeds the threshold voltage in the ReRAM, or the current exceeds the threshold current in the spin-injection MRAM, unintended rewriting of information (destruction of information) occurs. Therefore, the voltage applied and the amount of current flowing at the time of information reading needs to be controlled so as to be equal to or less than the threshold. However, when the threshold is different among elements, there is the concern of a voltage or current that was not a problem in one element causing the destruction of information in another element.
Furthermore, if there are variations in the resistance values in the low-resistance state and the high-resistance state in each element, it becomes difficult to judge whether each element is in the low-resistance state or the high-resistance state, and thus information cannot be read well. Therefore, in addition to controlling the voltage and amount of current needed for writing/rewriting, it becomes necessary to control the variation in element characteristics such as the resistance value for the low-resistance state and the resistance value for the high resistance state (hereinafter denoted simply as “variation control”).
(Methods for Power Consumption Reduction and Variation Control)
Two methods, namely, a method of forming a projection on an electrode, and a method of partially coating the surface of an electrode using an insulator, are well known as effective techniques for power consumption reduction and variation control.
The projection formed on the electrode causes electric charges to crowd in that part. As such, it is possible to start the flow of current using a lower voltage. Furthermore, since the probability that current will flow out from the projection is high, variations in current path can be suppressed. As such, the advantageous effect of suppressing element characteristic variation can be obtained. On the other hand, the length of the projection has very little to do with such effect.
Meanwhile, partial insulation coating of the electrode surface has the advantageous effect of limiting the current path to the portion that is not coated with the insulator. As such, current density rises in the uncoated vicinity of the electrode, and thus facilitating the occurrence of a change of state even with a lower amount of current. In other words, it becomes possible to rewrite information using a smaller amount of current. This advantageous effect is mainly an effect that is obtained after current starts to flow. Furthermore, since there is the advantageous effect of suppressing current path variation as in the projection, an element characteristic variation suppressing effect can also be obtained.
Although the forming of a projection on the electrode and the partial insulation coating of the electrode have partially overlapping functions as described above, these functions are not exactly the same. Therefore, the greatest advantageous effect can be obtained with a structure in which a projection is formed on the electrode and the entirety of the portion other than the projection is coated with an insulator.
International Publication No. 2005/041303 (Patent Reference 1), Japanese Unexamined Patent Application Publication No. 2006-203178 (Patent Reference 2), U.S. Pat. No. 5,155,657 Specification (Patent Reference 4), and Japanese Unexamined Patent Application Publication No. 2007-109821 (Patent Reference 6) disclose a method of forming a projection on an electrode. Furthermore, Patent Reference 1 and Japanese Unexamined Patent Application Publication No. 2008-159760 (Patent Reference 3) disclose a method for partial insulation coating of an electrode. In the case of a method in Japanese Unexamined Patent Application to Publication No. 2007-180473 (Patent Reference 5), a projection and an insulation coating are simultaneously formed.